In North America, semi tractors usually have 3 axles, the front, or 'steer' axle having two wheels, and each of the two rear 'drive' axles having a pair of 'dual' (double) wheels on each side. Thus, the most common configuration of tractor has 10 wheels. The cargo trailer usually has two 'tandem' axles at the rear, each of which has dual wheels, or 8 wheels on the trailer. The maximum weight for a tractor-trailer in this configuration is at least 80,000 lb (36.2 tonnes), although some states allow up to 99,000 lb (44.9 tonnes).
Although dual wheels are most common, use of a single, wider tire (known as 'super singles') on each axle is becoming popular, particularly among bulk cargo carriers and other weight-sensitive operators. The advantages of this configuration are two: the lighter weight allows a truck to be loaded with more weight, and the single wheel covers less of the brake unit, which allows faster cooling. The biggest disadvantage is that when a tire becomes deflated or destroyed, it is not possible to drive the vehicle to a service location without risking damage to the rim, as it is with dual wheels.
The United States also allows 2-axle tractors to tow two 1-axle 28.5-foot (8.7 m) semi-trailers known officially as STAA doubles and colloquially as doubles, a set, or a set of joints on all highways that are part of the National Network. Individual states may further allow longer vehicles (known as 'longer combination vehicles' or LCVs), and may allow them to operate on roads other than those part of the National Network.
LCV types include:
• Triples: Three 28.5 foot (8.7 m) trailers; maximum weight up to 129,000 lb (58.5 tonnes).
• Turnpike Doubles: Two 48 foot (14.6 m) trailers; maximum weight up to 147,000 lb (66.7 tonnes)
• Rocky Mountain Doubles: One 40 foot (12.2 m) trailer and one 28.5 foot (8.7 m) trailer (known as a 'pup'); maximum weight up to 129,000 lb (58.5 tonnes)
Regulations on LCVs vary widely from state to state. No state allows more than three trailers without a special permit. Reasons for limiting the legal trailer configurations include both safety concerns and the impracticality of designing and constructing roads that can accommodate the larger wheelbase of these vehicles and the larger minimum turning radii associated with them.
The long-haul tractors used in interstate travel are often equipped with a 'sleeper' behind the driver's cab, which can be anything from a small bunk to a rather elaborate miniature apartment.